This invention relates to the field of processes for preparing organomercaptans. More particularly, the present invention describes a process for preparing organomercaptan by the catalyzed hydrogenolysis of a disulfide, trisulfide, and/or polysulfide.
U.S. Pat. No. 6,147,242 describes a process for preparing 3-mercaptopropyl-triethoxysilane by the homolytic cleavage of the corresponding bis-disulfide. The method involves reacting a bis-silylalkylsulfide with an alkali metal and a chlorosilane to provide a silylalkylsulfanylsilane intermediate which is recovered and thereafter hydrolyzed in the presence of refluxing aqueous alcohol to the desired mercaptoalkylsilane. The foregoing method is subject to several disadvantages including its use of a chlorosilane which is expensive, the necessity to filter and dispose of hazardous alkali metal salt, and the need to isolate a silylalkylsulfanylsilane intermediate prior to the hydrolysis step.
Itabashi, CA 54, 2153e (1960), CA, 54, 19466c (1960) and CA, 54, 19466g (1960), describes the reaction of various disulfides with hydrogen over molybdenum (VI) sulfide catalyst at 130xc2x0-140xc2x0 C. and 10.7 MPa hydrogen pressure resulting in the hydrogenolysis of the disulfides at the Sxe2x80x94S linkage to provide the corresponding organomercaptans in high yields. The high catalyst loading for the reaction (approximately 5 weight percent) and the high cost of the catalyst both add significantly to the expense of this preparative method. A similar process described by Broadbent et al., J. Am. Chem. Soc., 76, 1519 (1954), provides thiophenol quantitatively by hydrogenolysis of diphenyl disulfide over Re2S7 in 2-methoxyethanol at 165xc2x0 to 195xc2x0 C. and 15 MPa hydrogen pressure. At these high temperatures, however, subsequent de-sulfurization exclusively results in the saturated hydrocarbon or aromatic substrate.
In general, precious metal and base metal catalysts have found little application in the selective cleavage of the Sxe2x80x94S bond due to the known poisoning effect of the resulting sulfides. In the few cases that have been reported, however, palladium catalysts, which are generally known for their resistance to catalyst poisons, have been the most reactive and have achieved the highest yields. The most striking example of this is the hydrogenolysis of methyl cystine to methyl cysteine in the presence of 25 weight percent of palladium catalyst in aqueous acid at room temperature and atmospheric pressure (Bergmann et. al. Ber. Dtsch. Chem. Ges. 63,987 (1930)). The necessity to use an unusually high loading of expensive palladium catalyst, however, precludes its use in all but a limited number of research applications.
Patent EP 649,837 discloses a process for the preparation of methyl mercaptan from the corresponding dimethyl disulfide using a transition metal catalyst which requires a sulfidation pretreatment with a hydrogen sulfide/hydrogen mixture (containing 15 mole percent hydrogen sulfide) at an hourly flow rate of 2 liters mixture per gram of catalyst at 400xc2x0 C. for 4 hours. The selectivity and the yield of the process are reported to be improved when the reaction is conducted in the presence of either water or hydrogen sulfide at a concentration of 0.1 to 15 weight percent with respect to the disulfide.
Other catalyst systems have also been reported that are based on transition metal sulfides since the sulfide phases are believed to be more resistant to poisoning by sulfur-containing molecules (Calais et al. J of Cat., 144, 160-174 (1993)). The use of platinum sulfides (Dutch Patent Application No. 6,402,424) for the reduction of diphenyl disulfide to phenyl mercaptan as well as the sulfides of Raney Ni and Raney Co (French Patent Application No. 2,008,331) and Ru, Rh, Pt, Ir, and Pd (German Patent Application No. DE 1,903,968) require relatively high hydrogen pressures, typically in excess of 5-10 MPa.
In accordance with the present invention, a process is provided for preparing an organomercaptan which comprises reacting a sulfide of the general formula (I) 
in which each R1 is the same or different alkyl group of up to about 6 carbon atoms, aryl group of up to about 10 carbon atoms or alkoxy group of up to about 6 carbon atoms, or at least two of R1 and the silicon atom to which they are bonded form a ring system having up to about 12 members and containing no ethylenic unsaturation, and optionally containing at least one heteroatom selected from the group consisting of oxygen, sulfur and nitrogen, each R2 is the same or different divalent hydrocarbon group containing no ethylenic unsaturation and having up to about 20 carbon atoms and m is 0 to about 8, with hydrogen under hydrogenolysis conditions in the presence of a catalytically effective amount of Group VIII metal catalyst and in the presence of a catalyst poisoning inhibiting amount of a catalyst poisoning inhibitory agent selected from the group consisting of water, except where the mercaptan product contains at least one hydrolyzable silane group, alkanol of from 1 to about 6 carbon atoms, hydrogen sulfide and mixtures thereof to provide organomercaptan product of the general formula (II)
(R1)3xe2x80x94Sixe2x80x94R2xe2x80x94SHxe2x80x83xe2x80x83(II)
in which R1 and R2 each has one of the aforestated meanings.
Unlike the catalyst sulfidation pretreatment required by the process described in EP 649,837, supra, the process of this invention does not require presulfiding in order to enhance reactivity or inhibit catalyst poisoning.
Under normal conditions, most base metals and precious metal catalysts are poisoned by the formation of sulfides and particularly by alkyl mercaptans. However, it has been discovered that when hydrogenolysis of sulfide is conducted in the presence of a catalyst poisoning inhibitory agent in accordance with this invention, the poisoning effect of the organomercaptan product can be minimized. As a result, both catalytic activity and selectivity increase substantially and high yields of organomercaptan product, e.g., in excess of 98%, can readily be achieved. The hydrogenolysis reaction herein has also been found to occur at more moderate temperatures and pressures. Low catalyst levels can be utilized and still provide completion in less than two hours with high conversion levels and excellent selectivity.
The starting sulfide of the present invention can be chosen from among those of the general formula (I): 
In sulfide (I), each R1 is the same or different alkyl group of up to about 6 carbon atoms and preferably of up to 4 carbon atoms, e.g., methyl, ethyl, propyl or butyl; aryl group of up to about 10 carbon atoms such as phenyl or naphthyl; alkoxy group of up to about 6 carbon atoms, and preferably up to 4 carbon atoms, e.g., methoxy, ethoxy, propoxy, isopropoxy, butoxy, or isobutoxy; or at least two of R1 and the silicon atom to which they are bonded form a ring system having up to about 12 ring members with no ethylenic unsaturation and optionally containing one or more oxygen, sulfur and/or nitrogen heteroatom members, e.g., the ring system having the structure 
each R2 is a divalent hydrocarbon group containing no ethylenic unsaturation and having up to about 20 carbon atoms, and preferably up to about 12 carbon atoms, e.g., a linear or branched alkylene group such as methylene, ethylene, 1,2-propylene, 1,3-propylene, 2-methyl-1,3-propylene, 3 methyl-1,3-propylene, 3,3-dimethyl-1,3-propylene, ethylidene or isopropylidene, a cycloalkylene group such as cyclohexylene or cycloheptylene, an arylene group such as phenylene, tolylene, xylylene or naphthylene, and m is 0 to 8 and preferably 0 to 4.
Reaction of sulfide (1) with hydrogen to provide organomercaptan product (II) in accordance with the invention can be thought of as proceeding in accordance with the reaction: 
Many examples of sulfide (1) that can be used for preparing organomercaptans in accordance with the present invention and methods for their manufacture are known in the art and include those disclosed in, e.g., U.S. Pat. Nos. 4,072,701; 4,408,064; 5,489,701; 5,466,848; 5,596,116; 5,663,395; 5,663,396; 5,859,275; 5,892,085; 6,147,241; 6,242,652; and, 6,274,755, the contents of which are incorporated by reference herein.
Examples of useful disilylalkyldisulfides of formula (I) include bis[3-(triethoxysilyl)propyl]disulfide[56706-10-6], bis[3-(trimethoxysilyl)propyl]disulfide[35112-74-4], 4,13-diethoxy-4,13-dimethyl-3,14-dioxa-8,9-dithia-4,13-disilahexadecane[188561-27-5], 4,4,13,13-tetraethoxy-6,11-dimethyl-3,14-Dioxa-8,9-dithia-4,13-disilahexadecane [89552-64-7], 8,11-dimethyl-5,5,14,14- tetrapropoxy-4,15-dioxa-9,10-dithia-5,14-disilaoctadecane[170573-44-1], 3,3,12,12-tetramethoxy-6,9-dimethyl-2,13-dioxa-7,8-dithia-3,12-disilatetradecane[170573-43-0], 3,3,12,12-tetramethoxy-4,11-dimethyl-2,13-dioxa-7,8-dithia-3,12-disilatetradecane[182814-38-6], 6,13-dimethyl-5,5,14,14-tetrapropoxy-4,15-dioxa-9,10-dithia-5,14-disilaoctadecane[182814-43-3], bis[3-(tributoxysilyl)propyl]disulfide[42168-82-4] and 5,14-diethyl-3,16-dimethyl-5,14-bis(1-methylpropoxy)-4,15-dioxa-9,10-dithia-5,14-disilaoctadecane.[180003-88-7].
Examples of useful disilylalkyltrisulfides of formula (I) include bis[3-(triethoxysilyl)propyl]trisulfide[56706-11-7], bis[3-(trimethoxysilyl)propyl]trisulfide[40550-17-2], bis[3-(triisopropoxysilyl)propyl]trisulfide[63501-63-3], 3,13-dibutyl-3,13-dimethoxy-2,14-dioxa-7,8,9-trithia-3,13-disilapentadecane[180003-90-1], 3-(tributoxysilyl)propyl 3-(trimethoxysilyl)propyl trisulfide [89552-63-6], (trithiodi-3,1-propanediyl)bis[tris(cyclopentyloxy)-silane[180003-75-2] and 5,21-diethyl-8,8,18,18-tetrakis[(2-ethylhexyl)oxy]-7,19-dioxa-12,13,14-trithia-8,18-disilapentacosane[180003-70-7].
Examples of useful disilylalkyltetrasulfides of formula (I) include bis[3-(trimethoxysilyl)propyl]tetrasulfide[41453-78-5], (tetrathiodi-3,1-propanediyl) bis [tris(isooctyloxy)-silane[180007-08-3], bis[3-triethoxysilyl)propyl]tetrasulfide[40372-72-3], 4,4-diethoxy-15,15-bis(ethoxymethoxy)-3,16,18-trioxa-8,9,10,11-tetrathia-4,15-disilaeicosane[167216-77-5], 6,6,17,17-tetrakis(ethoxymethoxy)-3,5,18,20-tetraoxa-10,11,12,13-tetrathia-6,17-disiladocosane[203457-58-3], 1,1xe2x80x2-(tetrathiodi-3,1-propanediyl)bis-2,8,9-trioxa-5-aza-1-silabicyclo[3.3.3]undecane[68704-61-0], bis[3-(diethoxymethylsilyl)propyl]tetrasulfide[70253-72-4], 2,17-dimethyl4,4,15,15-tetrakis(1-methylethoxy)-3,16-dioxa-8,9,10,11-tetrathia4,15-disilaoctadecane[63501-62-2], 4,4,15,15-tetraethoxy-7,12-dimethyl-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane[57640-08-1], 5,16,16-triethoxy-5-methoxy4,17-dioxa-9,10,11,12-tetrathia-5,16-disilaheneicosane[180003-77-4], 6,6,17,17-tetrabutoxy-5,18-dioxa-10,11,12,13-tetrathia-6,17-disiladocosane[57640-06-9], 3,3,14,14-tetramethoxy-5,12-dimethyl-2,15-dioxa-7,8,9,10-tetrathia-3,14-disilahexadecane[180004-00-6], 3,14-bis(1,1-dimethylethyl)-3,14-dimethoxy-2,15-dioxa-7,8,9,10-tetrathia-3,14-disilahexadecane[243458-27-7], disilatriacontane[57640-07-0], 10,10,21,21-tetrakis(octyloxy)-9,22-dioxa-14,15,16,17-tetrathia-10,21-disilatriacontane[180003-68-3], 10,21-diethoxy-10,21-bis(octyloxy)-9,22-dioxa-14,15,16,17-tetrathia-10,21-disilatriacontane[57640-13-8], 10,10,21-triethoxy-21-(octyloxy)-9,22-dioxa-14,15,16,17-tetrathia-10,21-disilatriacontane[57640-12-7], tetrathiodi-3,1-propanediyl)bis[tris(cyclohexyloxy)-silane[180003-74-1], 3,14-dimethoxy-3,14-diphenyl-2,15-dioxa-7,8,9,10-tetrathia-3,14-disilahexadecane[180003-91-2], 6,17-diethoxy-6,17-diphenyl-5,18-dioxa-10,11,12,13-tetrathia-6,17-disiladocosane[243458-31-3], 14-ethoxy-3,3-dimethoxy-14-phenyl-15-dioxa-7,8,9,10-tetrathia-3,14-disilanonadecane[180003-92-3] and 3,3,14,14-tetramethoxy-6,11-diphenyl-2,15-dioxa-7,8,9,10-tetrathia-3,14-disilahexadecane[137264-06-3].
It is well recognized that the known tetrasulfides are in fact average compositions, including a range from disulfide to octasulfide or higher, and typically are not pure tetrasulfides. Similarly, the useful disulfides and trisulfides can be provided as mixtures, the use of which is also contemplated herein.
The disilylalkyldisulfides are generally preferred due to generation of less by-product H2S, with the methoxy and ethoxydisilylpropyldisulfides being more preferred.
Hydrogenolysis conditions can include a hydrogen pressure from about 100 psig to about 1000 psig and preferably from about 300 psig to about 600 psig, a temperature of from about 160xc2x0 C. to about 200xc2x0 C. and preferably from about 180xc2x0 C. to about 190xc2x0 C., a reaction time from about 1 hour to about 5 hours and preferably from about 2 hours to about 3 hours.
The catalyst employed in the hydrogenolysis reaction is chosen from amongst the Group VIII metals and is preferably selected from the group consisting of nickel, cobalt, rhodium and ruthenium. The catalyst is preferably one that is supported on any one of numerous known and conventional catalyst support materials, e.g., diatomaceous earth, carbon, silica, alumina, aluminosilicate, and the like.
The amount of catalyst employed can vary widely provided of course, it is a catalytically effective amount. In general, catalyst levels of from about 0.1 wt. % to about 10 wt. % and preferably from about 0.5 wt. % to about 1 wt. % based on the weight of sulfide (I) reactant can be employed with good results.
The catalyst poisoning inhibitory agent of the present can be selected to be water (except where the organomercaptan product contains one or more water sensitive silane groups); an alkanol of from 1 to 6 carbon atoms, preferably selected to match the alkoxy group(s) R2 of the sulfide (I) reactant, and preferably one selected from the group consisting of methanol, ethanol, butanol and isobutanol; and, hydrogen sulfide. Catalyst poisoning inhibitory amounts of the catalyst poisoning inhibitory agent can vary widely and in most cases can be present in the reaction medium at a level of from about 5 wt. % to about 50 wt. % and preferably from about 5 wt. % to about 20 wt. % based on the weight of sulfide (I).
Comparative Examples 1-6 illustrate a catalyzed process for making organomercaptan which omits the use of a catalyst poisoning inhibitory agent and as such, are outside the scope of the present invention. Examples 1-27 are illustrative of the process for preparing organomercaptan of the present invention and clearly demonstrate the advantage of using a catalyst poisoning inhibitory agent. In the tables of data which accompany all of the examples, the following terms have the designated meanings:
xe2x80x9cMercaptanxe2x80x9d: 3-mercaptopropyltriethoxysilane
xe2x80x9cMonosulfidexe2x80x9d: bis(3-(triethoxysilylpropyl)sulfide
xe2x80x9cDisulfidexe2x80x9d: bis(3-triethoxysilylpropyl)disulfide
xe2x80x9cPolysulfidexe2x80x9d: mixtures of bis[3-(triethoxysilyl)propyl]trisulfide and higher sulfides
In a 1 liter Hastelloy C autoclave equipped with a mechanical stirrer, cooling coils and a thermocouple, 591.4 grams of mainly bis(3-triethoxysilylypropyl)disulfide were combined with 2.0 grams of a 55 wt % nickel catalyst on a kieselguhr support. After purging the autoclave with nitrogen and then hydrogen, the reactor was pressurized to 620 psig with hydrogen and heated to 190xc2x0 C. while stirring at 1022 rpm. After approximately 180 minutes, the reaction mass was cooled to room temperature and vented to atmospheric pressure. The contents of the reactor were sampled and analyzed by gas chromatography with the following results:
Employing substantially the same procedure as described in Comparative Example 1, additional reactions were carried out with the conditions and results as set forth in Table 2 below.
As the data in Tables 1 and 2 of Comparative Examples 1-6 amply show, absence of a catalyst poisoning inhibitory agent resulted in much larger amounts of disulfide product relative to the amounts of mercaptan produced.